In the packaging and tubular container art, packages have been formed by creating a tube having flexible sidewalls, formed of a thermoplastic material such as polyvinyl chloride (PVC) approximately 0.005 to 0.02 inches in thickness, held to a preformed or desired shape such as a polygon by coupling to the ends of the tube rigid end caps. The end caps employed in such packages or containers as evidenced by U.S. Pat. No. 3,986,659, issued Oct. 19, 1986, entitled "End Caps for Tubular Containers", and U.S. Pat. No. 3,913,774, issued Oct. 21, 1975, entitled "End Caps for Containers", are formed of a plastic material by molding to create rigid end members providing strength and rigidity to the package or container. The dimensions and shape of the end caps conform to the dimensions and shape of the flexible sides of the tube and the end caps include a base portion substantially forming an end cover for the container tube and sidewalls. The end caps also have engaging means for interlocking with perforations within opposed flexible sides of the tubes such that the projections extending outwardly of the sidewalls and within the perforations achieve a positive mechanical lock for holding the end caps to the container or package.
In an effort to improve and lower the cost of the container, end caps have been formed from identical or similar plastic sheet material as the tube proper by transverse severing short length sections from a prefold, flexible, semi-rigid strip of thermoplastic material such as polyvinyl chloride to the required end cap length. Such tubular package end cap is the subject matter of copending application Ser. No. 816,779, filed Jan. 7, 1986, and entitled "Tubular Package and End Cap for Same". In that application, the tubular package comprises a tube of sheet material of polygon cross-section having axially open ends, with the ends being closed off by suitable end caps. Each end cap is formed by a flexible semi-rigid sheet having a central portion spanning across the open end of the tube and terminating at opposite edges thereof and at right angles to the central portion in a double layer folded back portion towards the central portion defining at each side thereof parallel sidewalls defining a double layer slot for receiving the ends of respective opposed sidewalls of the polygon cross-section tube. The folded portions of the semi-rigid plastic defining the end cap outer sidewall are of a common height. In order to define a slot which is sized between the outer and inner sidewalls of the end caps equal to or very slightly smaller than the tightness of flexible tube sidewalls received thereby, the flexible, semi-rigid plastic sheet must be sharply folded under an arrangement which restores plastic memory at the crease or fold line so that the folded sheet portion strongly resists deformation and facilitates a relatively rigid connection between the end cap and the tube.
It is an object of the present invention to provide an improved method of forming a longitudinally folded thermoplastic material strip stock in which a thermoplastic material stock strip is heat softened only along desired bend lines to momentarily eliminate the memory of the plastic stock strip at selected longitudinal lines and wherein after bending, cooling of the heat-softened plastic restores the memory at the crease lines.
It is a further object of the present invention to provide a method and apparatus for longitudinally folding the plastic material stock strip, which may be of clear or translucent plastic and which is achieved without scratching the thermoplastic material during the process of localized line heating, bending and cooling by sequentially passing the stock through a heating station, a forming station and a chilled die crease line setting station.